Alpha-dystroglycanopathy is a subgroup of congenital muscular dystrophy (CMD) characterized by reduced or absence of O-glycosylation in the mucin-like domain in alpha-dystroglycan (alpha-DG) (Muntoni, F. (2004) Acta. Myol. 23(2), 79-84; Toda, T. (2005) Rinsho Shinkeigaku 45(11), 932-934; Muntoni, F., et al. (2007) Acta. Myol. 26(3), 129-135; Hewitt, J. E. (2009). Biochim. Biophys. Acta. 1792(9), 853-861; Godfrey, C., et al. (2011) Curr. Opin. Genet. Dev. 21(3), 278-285). The lack or hypoglycosylation on alpha-dystroglycan leads to the loss or decreased binding of its ligands, which include laminin-2, agrin and perlecan in skeletal muscle, neurexin in the brain, and pikachurin in the eye. Alpha-dystroglycan is a peripheral membrane component of the dystrophin-glycoprotein complex (DGC) (FIG. 1A) common to all muscles and the heart (Matsumura, K., et al. (1993) Neuromuscul. Disord. 3(5-6), 533-535). In these tissues, the DGC complex functions to link the filamentous actin (F-actin)—associated cytoskeleton of the muscle fiber via dystrophin to the extracellular matrix (ECM, also called basal lamina) via laminin-2 (FIG. 1B).
In alpha-dystroglycanopathies, mutations in at least 18 genes identified to date are linked to aberrant processing of O-glycosylation on alpha-DG and lack of binding to its ligands, leading to diseases. See, e.g., Hara, Y., et al. (2011) N. Engl. J. Med. 364(10), 939-946; Kim, D. S., et al. (2004) Neurology 62(6), 1009-1011; van Reeuwijk, J., et al. (2005) J. Med. Genet. 42(12), 907-912; Murakami, T., et al. (2009) Brain Dev. 31(6), 465-468; Yanagisawa, A., et al. (2009) Eur. J. Med. Genet. 52(4), 201-206; Clement, E. M., et al. (2008) Arch. Neurol. 65(1), 137-141; Endo, T., et al. (2010) Methods Enzymol. 479, 343-352; Saredi, S., et al. (2012) J. Neurol. Sci. 318(1-2), 45-50; Longman, C., et al. (2003) Hum. Mol. Genet. 12(21), 2853-2861; Lefeber, D. J., et al. (2009) Am. J. Hum. Genet. 85(1), 76-86; Barone, R., et al. (2012) Ann. Neurol. 72(4), 550-558; Toda, T., et al. (2003) Congenit. Anom. (Kyoto) 43(2), 97-104; Toda, T. (2007). Rinsho Shinkeigaku 47(11), 743-748; Puckett, R. L., et al. (2009) Neuromuscul. Disord. 19(5), 352-356; Toda, T. (2009). Rinsho Shinkeigaku 49(11), 859-862; Yamamoto, T., et al. (2010) Cent. Nerv. Syst. Agents. Med. Chem. 10(2), 169-179; Kanagawa, M., et al. (2016) Cell. Rep. 14(9), 2209-2223; Yoshida-Moriguchi, T., et al. (2013) Science 341(6148), 896-899). These genes include, for instance, many glycosyltransferases, such as LARGE, which encodes a xylosyl- and glucuronyl-dual transferase responsible for adding xylose-glucuronic acid repeats to glycans to facilitate ligand binding (Inamori, K., et al. (2012) Science 335(6064), 93-96; Longman, C., et al. (2003) Hum. Mol. Genet. 12(21), 2853-2861). The main biological function of glycosyltransferases in this pathway (e.g. LARGE) are to properly assemble the O-glycosylation in the mucin-like domain in alpha-DG, which is necessary for tight binding to laminin-2 in the basal lamina of muscles, agrin and perlecan in neuromuscular junction, neurexin in the CNS, and pikachurin in the eye (Michele, D. E., et al. (2002) Nature 418(6896), 417-422; Muntoni, F., et al. (2002) Lancet 360(9343), 1419-1421). In the absence of proper O-glycosylation due to a defect in any of the aforementioned genes, binding of alpha-DG to laminin-2 in the extracellular matrix (ECM) is compromised or lost (FIG. 1C), causing a breakage of the mechanical link that is necessary for sarcolemma integrity. This renders the muscles prone to contraction-induced injury, resulting in damage to the sarcolemma of the muscle fiber and consequent muscular dystrophy (Barresi, R. and Campbell. K. P. (2006) J. Cell. Sci. 119, 199-207).
Due to the genetic heterogeneity, alpha-dystroglycanopathies include many subtypes of diseases which exhibit diverse yet overlapping clinical manifestations from very severe muscular dystrophy with central nervous system (CNS) and eye abnormalities to relatively mild muscular dystrophic phenotype without CNS manifestation or eye problem. There is no strict genetic and phenotypic correlation between different subtypes of alpha-dystroglycanopathies. Mutations in one gene can cause different subtypes of diseases with overlapping clinical manifestations, and mutations in different genes may lead to the same or similar disease (Godfrey, C., et al. (2007) Brain 130, 2725-2735). Because of this heterogeneity, strategies to treat individual alpha-dystroglycanopathies caused by mutations in individual genes have not been attractive for drug development due to the low cost effectiveness.
Alpha-dystroglycan and beta-dystroglycan are encoded by the same gene DAG1 and translated from a single mRNA as an intact type-1 transmembrane protein, dystroglycan. En route to the cell surface, dystroglycan is proteolytically cleaved to generate the transmembrane stud beta-dystroglycan and the noncovalently associated alpha-dystroglycan (Holt, K. H., et al. (2000) FEBS Lett. 468(1), 79-83). Theoretically, recombinant alpha-dystroglycan with proper O-glycosylation has been proposed as a protein replacement therapy for alpha-dystroglycanopathies. However, systemic delivery of recombinant alpha-dystroglycan indicated that this protein failed to reach the muscle interstitial space to be incorporated onto to the sarcolemma (Han, R., et al. (2009) PNAS 106(31), 12573-12579). Utilizing recombinant alpha-dystroglycan as protein replacement therapy for alpha-dystroglycanopathies is therefore thought to be technically impractical.
Therefore, a need exists for therapeutic molecules for preventing and/or treating alpha-dystroglycanopathies and their associated pathologies.
All references cited herein, including patent applications, patent publications, and UniProtKB/Swiss-Prot Accession numbers are herein incorporated by reference in their entirety, as if each individual reference were specifically and individually indicated to be Incorporated by reference.